Meeting Abstract
Testing ecogeographical rules, which posit strong relationships between organismal phenotypes and habitat, can help us understand evolutionary mechanisms that generate biodiversity. Although not formally named an ecogeographical rule, environments at different ocean depths vary in predictable ways important to organisms living there. Our knowledge of these factors, coupled with our understanding of metabolism and visual ecology, predicts strong associations between organismal phenotypes and oceanic depth. A valuable group to study these associations is cylindroleberidid ostracods (Crustacea). We used comparative methods to ask how habitat depth is related to eye morphology and body size (carapace length) in cylindroleberidids because of previous phylogenetic analyses, and their enormous depth range. We collected and analyzed data for 128 species, including 37 that lack eyes. For each, we recorded habitat depth, body size, diameter of the largest ommatidium in the eye, eye size, and number of ommatidia per eye. We find no evidence for the general prediction that body size decreases with depth due to lower food availability. We also find that eye length and ommatidium diameter are not related to depth, counter to a prediction that species living at greater depths have larger eyes that gather light more efficiently. Defying simple explanation, we find strong evidence that ommatidia count decreases with depth in the photic zone, but we see the opposite trend in the dysphotic zone. Finally, we find as expected a significantly higher proportion of eyeless species with depth. Taken together, our results indicate that morphological changes along ecogeographical gradients do not follow simple relationships. Instead, exceptions to simple predictions of morphological changes sometimes depend on interactions between characters, phylogeographic history, and non-linear relationships between habitat gradients and characters.
